In recent years there have been numerous attempts to develop damage detection and condition assessment capabilities that could be used to mitigate hazards associated with potential failure of bridges, as well as other large civil structures. The need for the development of these techniques is motivated by the estimated tens of billions of dollars needed to repair or retrofit these structures. These efforts have been based, in part, on modal testing techniques that have attempted to identify critical response indices sensitive to changes in structural condition or integrity. Most approaches that have been demonstrated to show sensitivity to changes in structural condition have required extensive field measurements often requiring bridge closure to traffic. Early work focused on modal indices indicating changes in modal resonant frequencies as an indicator of potential damage. However, recent work has revealed these modal indices to be largely inadequate due to the weak correlation between damage and measured frequency shifts. The use of damping has also not provided reliable indications of damage in most cases. Changes in mode shape geometry, that is, changes in the vibrating shape of the structure at a resonant frequency, have shown greater sensitivity to damage but lack practical accuracy. Other more sensitive methods of using modal parameters to assess changes in structural condition have been developed, including the use of flexibility, strain energy, and mode shape curvature, for example. These approaches do not, however, provide a quick and accurate description of the overall condition of the bridge that might enable prompt inspections and early corrective actions.
Ambient measurements for a particular bridge, the Seymour Bridge of Cincinnati Ohio, have been made to provide initial characterization of the bridge for baseline comparisons. This particular bridge is approximately 40.0 m long and 12.0 m wide from the proximal bridge end to the distal bridge end, with 3.7 m wide traffic lanes and 2.4 m wide sidewalks for pedestrian traffic. Two bridge piers provide support for the deck located 12.0 m inward from abutments at the proximal and distal ends. The upper deck surface is a composite design made of 6.4 cm of asphalt surface over a 16.5 cm thick concrete slab and supported by six steel I-Beams girders all of which extend along the length of the bridge. The exterior beams under the sidewalk edges are W21.times.73 and the interior beams are W27.times.94. Lateral bracing is provided by intermediate cross frames that are spaced approximately 3.67 m apart. The bracing used is L section 3.times.3.times.5/16 welded with 0.64 cm fillet welds to the beam webs. Ambient acceleration responses are acquired on the bridge monitoring locations using accelerometers mounted on small aluminum blocks bolted to a leveling platform that stands off the deck surface on three leveling screws. Each mounting block contains at least one accelerometer oriented in the vertical direction, but may also contain a second accelerometer oriented in the transverse direction. The accelerometer platform can be simply placed on the upper deck surface and a bubble level used to ensure true vertical and transverse orientation for each measurement axis. No bonding agents or adhesives are required for accelerometer placement on the deck surface. Typical ambient measurement locations are spaced apart, such as, at 3.0 m intervals along the length of the bridge and at 2.6 m along the width, for a total of 90 monitor locations during testing. Bi-axial acceleration measurements were oriented along longitudinal through-traffic and transverse cross-traffic directions at each location. During the ambient tests, it is necessary to locate two fixed measurement locations that are later utilized as reference measurements in the computation of the frequency response. Although ambient responses may contain large transient spikes corresponding to vehicular traffic passing under the bridge, these spikes do not typically exceed a 10.0 mg peak. Transient components of the ambient responses usually decay within one second. Random vibration levels present in the responses are bounded by a 1.0 mg peak, and accurate levels can be determined to be less than a 0.1 mg peak. Power spectral estimates show that although the responses along the transverse directions were substantially smaller in magnitude, sufficient signal quality was present for correlation with vertical measurements. Ambient testing can provide power spectral measurements that are used to identify modal frequencies and shapes, such as resonance frequencies at 7.1 Hz and 8.2 Hz for the Seymour Bridge.
Analyses of the ambient responses revealed that, although changes in bridge modal amplitude response characteristics could be measured and perhaps attributed to the presence of damage in the bridge, these changes were limited to the order of 10%. In the context of measurement uncertainties that may approach 10%, the sensitivity of this modal amplitude index appears inadequate. During the conduct of the ambient tests, noticeable and measurable differences in wave arrival times can be observed on the bridge from traffic induced transient responses. However, wave speed indices that might provide some correlation to changes in structural condition on the steel stringer bridge, have not been used to reliably determine bridge damage. These and other disadvantages are solved or reduced using the invention.